The funeral of Mark Duggan, whose death is the subject of an IPCC investigation, September 2011. Photograph: Graeme Robertson

I have been engaging with the police since they accepted the findings of the Macpherson report, which identified institutional racism within the force. I became an independent (and unpaid) adviser to Operation Trident, the Met's specialist unit on so-called black gun crime. I also helped to set up the Black Independent Advisory Group, which meets with police locally, and chaired it for its first two years.

I engage with the police for one purpose: to improve the policing provided by the Met to the black community. I am no supporter of riots; a night of "payback" might seem like a good idea at the time, but it brings no long-term benefit to those who participate or to the area, which inevitably gets destroyed and further stigmatised.

Engaging with the Met when you come from a place like Tottenham has risks. Not least that you will be seen as a snitch or a grass by some within the community. To overcome this I have tended to look at past police operations rather than ongoing investigations. This way I can advise the police on how to handle similar inquiries in future, in a way that might involve and empower communities so they become willing partners, instead of cynical onlookers.

I imagine that most officers who have worked with me would describe the experience as very challenging. I make no apology for this. When Assistant Commissioner Cressida Dick left her position as the operational commander for Trident she took me aside and said: "You are hard work, but do not stop doing what you do, the Met needs people like you to hold us to account!"

When the Independent Police Complaints Commission set up a community reference group (CRG) to oversee the investigation into the shooting of Mark Duggan I put myself forward. I believed that my background not only gave me credibility within the community, but would also ensure that the IPCC would do all it could to carry out a rigorous, transparent investigation. I signed a confidentiality agreement that I have honoured up until now. However, I have been compelled to resign from the CRG as the IPCC's investigation into the shooting of Duggan is unlikely to meet any of the criteria I set out.

The IPCC has broken its own guidelines by giving out erroneous information to journalists regarding the "shoot-out" involving Duggan and police that didn't actually happen. And its investigation is flawed and in all probability tainted – so much so that we can never have faith in its final report.

I discovered, within the community, that the minicab from which Duggan was taken seconds before he was shot, had been moved from the scene. This was long before the arrival of the IPCC investigators. When I brought this to the IPCC commissioner's attention she confessed to not knowing anything about it. This was some two months into the investigation. We were later informed by her that the police had wanted to give the vehicle back to its owner, and there was no forensic data on it.

It was later revealed that there was, in fact, forensic and other evidence, of major significance, in and on the vehicle. We were also told that the IPCC investigators had demanded it be brought back to the scene. Weeks after these revelations, we were told by a very embarrassed IPCC commissioner that her own investigators had sanctioned the removal of the vehicle – before they had even reached the crime scene.

It took three months for this to be revealed. It also took three weeks for the IPCC to realise that it had wrongly told journalists that police had been involved in a shoot-out with Duggan. We all know the impact that piece of misinformation has had, not only on Tottenham but on the whole country.

I believe that in removing the minicab from the scene, the police have clearly compromised the evidence. Given that the IPCC sanctioned the vehicle's removal, the entire investigation is now in doubt.

There is one final piece of outstanding investigative work that we need to put into the public arena. We were told that at least three officers had given a statement that they had witnessed another officer, a sergeant, throwing away the gun that was later found several feet from Duggan's body. When we sought to identify the officers involved, we were told there was no evidence to support the allegation – that this statement was in fact never given. Yet we were informed by the commissioner that it had been.

Last week a CRG meeting took place in the IPCC offices in central London. The commissioner wanted us there so she could explain away the concerns we had been raising. I and another CRG member, John Noblemunn, did not attend because we knew what would happen. The commission would seek to explain the inexplicable, or try to set up yet another investigation where they investigate themselves, which will then seek to explain the inexplicable.

In all of my years of engagement with the Met, I have yet to witness such a shoddy investigation. It is time we recognise that we need a body truly willing and able to investigate the police. This is the only way to ensure that they learn from their mistakes; and that, when mistakes occur, communities do not believe the one route to justice lies in taking matters into their own hands.

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